Ossan Boukensha no Okureta Eiyuutan: Kansha no Suburi wo 1-nichi 1-mankai shiteitara, Kensei ga Deshiiri Shigan ni Yattekita

Guy Gullveig was abused by his half-brother because he was born as an illegitimate child. He was taken in by his grandfather, a former adventurer who lived deep in the mountains, and began training his body and mind by practicing "sword swings." Even after his grandfather passed away, Guy continued doing 10,000 practice swings every single day, and before he knew it, he had grown into a middle-aged man. One day, Guy finally leaves the mountains and heads to the city to become an adventurer, just as his grandfather once recommended. But there was one thing Guy didn't know... Thanks to the countless practice swings he had continued all this time, he had already become the strongest swordsman.

(Source: Square Enix, translated)